1. Field of the Invention
This invention relates to an intraocular length measuring instrument in which the position of a corneal vertex is found by using an optical system relating to geometrical optics, the position of the surface of an intraocular object to be measured is found by using an interferometric system relating to physical optics, and the intraocular length between the positions of the corneal vertex and the object surface is measured.
2. Description of the Prior Art
A conventional intraocular length measuring instrument well-known in the art includes a pattern image projecting and receiving optical system and an interferometric system. In the pattern image used projecting and receiving optical system, a ring image as a pattern image is projected onto the cornea of a subject's eye and then received into a two-dimensional image sensor. In the interferometric system, a measuring beam of light is projected onto the surface of an intraocular object to be measured while a reference beam of light is projected onto a reference surface corresponding to the surface of the object, interference is then caused between the respective beams of light reflected from the object surface and the reference surface, its resultant interference light is guided to a photosensor, and an interference signal is output from the photosensor. Finally in the intraocular length measuring instrument, a corneal vertex position is found by the pattern image projecting and receiving optical system, an object surface position is found by the interferometric system, and the intraocular length between the corneal vertex and the object surface is calculated from those positions.
In this conventional instrument, the photosensor provided in the interferometric system outputs a maximum amplitude waveform and, immediately after that, data concerning the ring image received by the two-dimensional image sensor is memorized in a frame memory used as a memory medium. Then the corneal vertex position is determined from the memorized data while the position of the object surface is determined from the interference signal obtained by the interferometric system.
However, the conventional instrument has a fault in that there is a time-lag of at least 1/30 seconds or so between the ring image data for determining the corneal vertex position and the data for determining the object surface position. If the eye blinks or is involuntarily moved during this time-lag, the corresponding relationship between those data is disturbed greatly and this disadvantageously leads to an inaccurate measurement of the intraocular length.